Documentation: bcachefs: SubmittingPatches: Demote section headings

SubmttingPatches.rst has 4 section headings, all under the same heading
levels. In absence of title headings, these section headings are all
ended up as title headings in the docs output, which also affect
the index toctree (increasing titles to 6 from the original 2)
due to :numbered: option.

Demote second-to-last section headings, making "Submitting patches
to bcachefs" as title heading.

Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>